Pressure instruments that serve as a background for the invention can be grouped into three categories: general purpose pressure instruments; manometers; and piston gauges. General purpose pressure instruments are devices such as Bourdon tube pressure gauges, capacitance diaphragm gauges and pressure transducers which are used to monitor/measure process parameters or as secondary standards used in the calibration of process equipment. Bourdon tube gauges use an elastic tube which flexes as a function of the applied pressure. Capacitance diaphragm gauges use a moveable membrane which varies the capacitance of the sensing element as a function of the applied pressure. Pressure transducers use a strain gauge which changes value as a function of the applied pressure. These instruments either use elastic elements or moving parts.
Manometers are liquid filled devices which measure pressure as a function of the change in height of the column(s) of the liquid. These devices can use water, alcohol, benzine, mercury or other fluids as the measurement medium. The difference in the column heights is monitored with a scale or ruler calibrated in the pressure units of interest. In the most accurate namometers, lasers have been used to measure the column heights. In this fluid based measurement system, it is necessary to change fluids at specified intervals. Because alcohol, benzine and mercury are all RCRA (Resource Conservation Recovery Act) regulated hazardous materials this procedure produces hazardous wastes.
Piston gauges are pressure measurement systems that measure pressure based on the cross sectional area of a piston and an applied mass. These instruments are typically used as primary pressure standards due to the fact that the measurement is based upon the physical quantities of mass and area. During use, a mass consisting of one or more certified weights is placed upon a weight table which is attached to the piston. The applied pressure is then increased or decreased through the use of valves and volume adjusters in order to place the piston and mass on a float, the term float referring to a point when the piston is free to move and encounters no friction forces from the upper or lower physical restraints used to hold the piston within its cylinder. Piston gauges of this caliber are manufactured to extremely tight tolerances. As with the other gauges, piston gauges contain moving parts.
Thus it is an object of the invention to provide a pressure measuring means free from moving parts or elastic elements.
It is another object of this invention to provide a pressure measuring means that reduces, or ultimately eliminates, the use of hazardous waste materials.
It is yet another object of the invention to provide a pressure measuring means that does not require the use of weights to measure pressure.
These objects may be achieved by providing a pressure monitoring apparatus that utilizes changes in laser light energy as a result of modulations or changes in atmospheric pressure to accurately measure pressure within a system or environment.